1. Field of the Invention
The present invention relates to an exhaust heat recovery apparatus that recovers thermal energy from the exhaust gas of an internal combustion engine, factory exhaust heat, or the like to produce motive power.
2. Description of the Related Art
An exhaust heat recovery apparatus is available that, using a heat engine, recovers the exhaust heat from an internal combustion engine that is mounted on a vehicle, such as a passenger car, a bus and a truck. In the exhaust heat recovery apparatus used for such a purpose, an external combustion engine is used, such as the Stirling engine, which is excellent in theoretical thermal efficiency, for example.
Published Japanese Translation of PCT application No. 2003-518458 (JP-T-2003-518458) discloses a technology in which a clutch is provided between the internal combustion engine and the Stirling engine, and the Stirling engine is used as an additional drive unit of the internal combustion engine. In addition, Japanese Patent Application Publication No. 2004-332665 (JP-A-2004-332665) discloses a technology concerning a system that includes a combustion engine, an electric generator, a motor, and a Stirling engine for driving the electric generator, wherein the electric power generated by the electric generator is supplied to the motor and a battery, and the electric generator is driven by the motor, wherein, a clutch is provided between the electric generator and the combustion engine, and the electric generator is driven by the combustion engine or the motor in a selective manner using the clutch.
In external combustion engines, such as the Stirling engine, the heat input from a heat source, such as the exhaust gas, is transferred to the working fluid of the external combustion engine through a heat exchanger, and the output from external combustion engines is therefore less responsive to the variation of the heat input. In addition, external combustion engines, such as the Stirling engine, which obtain thermal energy from a heat source through a heater, sometimes continue to produce motive power even after the heat input is stopped, by virtue of the heat stored in the heater.
As a result, if an external combustion engine, such as the Stirling engine, is used as the exhaust heat recovery unit, the exhaust heat recovery unit sometimes continues to produce motive power by virtue of the heat stored in the heater that the exhaust heat recovery unit includes, even when exhaust heat is not supplied to the exhaust heat recovery unit. In such a case, in the case of the configuration in which the output from the exhaust heat recovery unit and the output from the heat engine are combined and output together, the exhaust heat recovery unit produces surplus motive power even when there is no request to drive the heat engine. In JP-T-2003-518458, the poorness of the output response when an external combustion engine, such as the Stirling engine, is used as the exhaust heat recovery unit is not considered, and there is yet room for improvement.
In the meantime, with regard to the technology disclosed by JP-T-2003-518458, the configuration is adopted in which an electric generator is always interposed. When the thermal energy recovered by the exhaust heat recovery unit is output as electric energy, the energy conversion efficiency of electric generators and batteries is less than 100%. Thus, if the technology disclosed by the JP-T-2003-518458 is used, the thermal energy recovered by the exhaust heat recovery unit declines.